The invention relates to direct AC-AC converters, i.e., frequency changers such as are being controlled to change AC electric power from one frequency to another through a single stage of conversion. This is in contrast to a two-stage system involving an intermediate DC link connected to an inverter. Direct AC to AC conversion offers significant advantages over the DC link approach to frequency changing. As shown in the prior art, the direct AC to AC converter may be an unrestricted frequency changer (UFC) like shown in U.S. Pat. Nos. 3,470,477 and 3,493,838 of Gyugyi, or a matrix converter as disclosed in the cross-referenced copending patent applications. Generally, the direct AC/AC converter may look as a matrix converter owing to the fact that a plurality of bilateral switches look like a grid of switches mounted between an AC input and an AC output which are controlled as a "matrix" to effect a desired voltage wave reconstruction. Therefore, the direct AC/AC converter will also be referred to hereinafter as a matrix-type converter, without any limitation being implied by this wording.
Under control in the matrix to connect input lines to output lines directly according to a predetermined control scheme, the bilateral switches are alternately switched ON and OFF under a phase line voltage. The switch may consist of a bipolar transistor, a gate turn-off thyristor (GTO), or a force-commutated thyristor surrounded by bridge rectifiers and auxiliary components. Whatever the type of switching device, commutation requires the provision of circuitry involving a capacitor, and components like diodes and resistors to facilitate the transition during commutation from one state to the next, through pulse shaping, for loss reduction or stress minimization. Such a circuit is known as a "snubber".
Energy accumulated in the capacity of the snubber and dissipated through the associated resistor is a loss which is not at all negligible, considering the high repetitive rate of the commutation steps. Also, the requirement to maximize the snubber action upon the associated switch calls for an increased capacitance, whereas the high rate of switching desirable goes the other way.